A product Roadmap is a guidepost which shows how the product will be developed. It is a visual depiction of product releases. It communicates what would be the main components in each release. It is a quick view of release points with the expected functionality. This is a high-level planning tool.

One important point: In Agile we are ready to accept changes, so for planning in each release, you have to go back to the roadmap to check if plan still work or need adjustments.

"artifact that contains release plans as its first level of decomposition"

Since it is high leval - It describes at high level how the product will likely to grow. It says what would be the main components in the product. It helps to develop Product Backlog, Release backlog and Sprint Backlog in chain.